C6x assembly programming  (Page 2/11)

Every instruction in the C6x can execute conditionally. There are five registers available for conditions (on the C67x processors): A1, A2, B0, B1, and B2. If blank, the instruction always executes. Conditions can take a form such as [A1] where the instruction will execute if A1 is not zero. This can be handy for making loops were the counter is put in a register like A1 and is counted down to zero. The condition is put on the branch instruction that branches back to the beginning of the loop.

Register files

Where are the numbers stored in the CPU? In the C6000, the numbers used in operations are stored in register. Because the registers are directly accessible though the data path of the CPU, accessing the registers is much faster than accessing data in the external memory.

The C6000 CPU has two register files (A and B). Each of these files consists of sixteen 32-bit registers (A0-A15 for file A and B0-B15 for file B). The general-purpose registers can be used for data, data address pointers, or condition registers. The following figure shows a block diagram of the C67x processors. This basic structure is similar to other processors in the C6000 family.

The general-purpose register files support data ranging in size from 16-bit data through 40-bit fixed-point. Values larger than 32 bits, such as 40-bit long quantities, are stored in register pairs. In a register pair, the 32 LSB's of data are placed in an even-numbered register and the remaining 8 MSB's in the next upper register (which is always an odd-numbered register). In assembly language syntax, a colon between two register names denotes the register pairs, and the odd-numbered register is specified first. For example, A1:A0 represents the register pair consisting of A0 and A1.

Let's for now focus on file A only. The registers in the register file A are named A0 to A15. Each register can store a 32-bit binary number. Then numbers such as a , x , prod and y above are stored in these registers. For example, register A0 stores a . For now, let's assume we interpret all 32-bit numbers stored in registers as unsigned integer. Therefore the range of values we can represent is 0 to $2^{\text{32}}$ −1. Let's assume the numbers a , x , prod and y are in the registers A0, A1, A3, A4, respectively. Then the above assembly instructions can be written specifically:

The TI C6000 CPU has a load/store architecture. This means that all the numbers must be stored in the registers before being used as operands of the operations for instructions such as MPY and ADD . The numbers can be read from a memory location to a register (using, for example, LDW , LDB instructions) or a register can be loaded with a constant value. The content of a register can be stored to a memory location (using, for example, STW , STB instructions).

In addition to the general-purpose register files, the CPU has a separate register file for the control registers. The control registers are used to control various CPU functions such as addressing mode, interrupts, etc.